Here is a guide to replacing your analogue clock with an 18 Button OBC. It's actually pretty easy but you should take your time and only do 1 wire at a time or it can quickly become confusing.

If you don't have air con or climate control you only need to worry about 11 wires, if you do have air con/climate then you need 12 wires. Total 14 if your adding the temp sensor also.

It's broken down like this (assuming no air con):

3 wires from the obc go to white plug (x16)
5 wires from the obc go to blue plug x17 (3 of these there are no wires for and need to go straight into the connectors using the 0.1in Socket Terminals)
3 wires from the obc go to the 3 wires left from the analogue clock
1 wire goes under the steering wheel

DISCLAIMER: I take no responsibility if you attempt this and break something.
NOTE: I don't take full credit for this guide as most of the information is out there I just gathered the most comprehensible bits from all the guides I could find and stuck them together with my own photos, all credits at bottom of the guide.

I also didn't add the Temp Sensor or Check Control module and sensors so the following features are not working:

Temperature, Check Control, Code.

---------------------------------------------------------STEP 1:

Preparation. Make sure you have everything you need ready before you start.

Add all the wires required from the connectors X1070 and x1071 to the terminal blocks while referencing the wiring diagram you can do this part inside, I put them in a block together in order except for the VIO/WHT wire which goes under the steering column.

Step 3:DISCONNECT THE BATTERY

Remove the analogue clock, the anti slip tray should pull out easily then put something underneath and pull it forwards (I used a panel removal tool).

Then take out the cigarette lighter shelf part below.

Step 4:

Remove the instrument cluster there are 2 torx screws to remove then use a panel removal tool or something similar at the top it should come out with a bit of wiggling.

Disconnect the 3 plugs at the back of the cluster.

It's possible to then squeeze the cluster out to the left without removing the steering wheel but it is quite fiddly.

Step 5:

Time to start splicing and tapping start with the wires from the Black OBC Connector (x1070) that go to The Blue Connector (x17) from the Instrument Cluster working your way down through the wiring diagram.

Step 5A/B:
As 3 of the wires that go to Blue plug (x17) don't have a wire to splice into you need to use the Socket Terminals and tap directly into the connector. To get to the connector pull the white clips off and the connector will slide right out of the plug:

Repeat for the wires from the White Connection of the OBC (x1071)

Step 6:
Tap into the final wire located under the steering wheel. There are 3 Philips screws that need to be removed, to get the panel off. Your looking for a Purple/White wire the same size as all the others.

Step 7:
Put the instrument cluster back in and reconnect the 3 plugs, connect the OBC up to the 2 connectors. Reconnect the Battery. Put your key in and switch the ignition to the on position.

If the OBC Lights up give yourself a pat on the back for a job well done just need to screw and clip everything back in now.

CREDITS:
Lys for the Wiring Diagram I simply changed it from Italian to English: http://www.lys.it/bmw/3.html
Daz Ultra for the most relevant/complete guide I could find.
Pelican Parts for another useful guide.
Shoulders for the spreadsheet detailing which wires were for what and also the ones that weren't present made it much easier to figure out which wires would be required and which could be ignored: http://www.mediafire.com/file/dytyzmeetmo/obcwiring.xls